Boreholes or wells are drilled in earth formations for exploration, and for stimulation and production of hydrocarbons. Boreholes are usually drilled along predetermined paths and the drilling of a typical borehole proceeds through various formations and stratigraphic layers. A drilling operator typically controls surface-controlled drilling operational parameters, such as the weight on bit, drilling fluid flow rates, drill string rotational speeds and drilling fluid parameters (e.g., the density and viscosity of drilling fluid). Downhole operating conditions can continually change and the operator must react to such changes and adjust operational parameters to optimize a drilling operation.
It is important to obtain information regarding the characteristics and properties of a formation prior to and/or during drilling, so that drilling operations can be designed and changed to optimize drilling. For example, different types of layers in a formation may require different operating parameters to maintain safety levels while preserving acceptable drilling times. Knowing in advance when a drill bit will start cutting into a certain formation or layer can be important to prevent hole instability or other drilling problems, and/or to adjust completion schemes. Information regarding formation layers and other properties is also important for other operations such as reservoir navigation. Knowledge of the positions and properties of stratigraphic layers facilitates well planning and also facilitates landing components at optimal locations.